1. Field of the Invention
This invention relates to an improved insulating layer, and, more particularly, to a method of forming an insulating layer of a ladder-type silsesquioxane copolymer, derived from an aminoalkoxysilane and an arylalkoxysilane, and which demonstrates improved planarizing and thermal stability properties.
2. Description of the Prior Art
In the field of semiconductor processing, it is well known to use organic materials as insulating layers. In particular, polyimide resins have been widely used for this purpose. However, it has been found that the polyimide resins, which have been used, while suitable in many respects, tend to exhibit rather poor planarizing characteristics over topographic obstructions. Because of these and other problems, the use of other organic materials, such as organosilanes, has been investigated.
For example, in IBM Tech. Discl. Bull., Vol. 28, No. 12, p. 5246 (May 1986), a polysiloxane dielectric is formed on a polyimide layer by spin-applying and then heat treating a hydrolyzed organosilane monomer (e.g., an aqueous solution of 3-aminopropyltriethoxysilane).
Also, in U.S. Pat. No. 4,480,009, issued to Berger on Oct. 30, 1984, selected polysiloxane units are incorporated into polyimides for the purpose of forming protective coatings with improved adhesion properties.
See also U.S. Pat. No. 4,222,792, issued to Lever et al. on Aug. 15, 1982, where an organosilane (i.e., a polysiloxane) is used as a fill material in the formation of planar, deep isolation trenches.
However, it has been found that the polysiloxanes, while generally having planarizing characteristics which improve upon the polyimides, have demonstrated stress-induced cracking and relatively poor adhesion.
Recently, another type of organosilane--namely, a silsesquioxane polymer--has been introduced into the art for use as an insulating layer. For example, in U.S. Pat. No. 4,349,609, issued to Takeda et al. on Sept. 14, 1982, cured coatings of non-amino-containing ladder-type silsesquioxane polymers, either alone or in admixture with silanol or titanol compounds, are employed as interlayer insulating layers between metal wiring layers.
There are a number of problems, however, with the approach in Takeda et al. For instance, such systems must be stored at low temperatures to retard aging, and they have a limited shelf-life despite such precautions, leading to increased costs and process control problems in use. Also, the addition of silanol starting materials into the system tends to degrade the dielectric properties of the resulting layer.
U.S. Pat. No. 4,626,556, issued to Nozue et al. on Dec. 2, 1986, also discloses the formation of a non-amino-containing silsesquioxane polymer layer. According to the process in this patent, water is reacted with a trihalogenosilane in the production of an organopolysilsesquioxane, which is used in a mixture with a compound which generates crosslinking-reaction-active species upon irradiation, in the formation of a patterned insulating layer.
In U.S. Pat. No. 4,723,978, issued to Clodgo et al. on Feb. 9, 1988, an organoglass insulating layer is produced by first forming a modified ladder-type silsesquioxane polymer from a silanol solution, and then treating it in an oxygen plasma.
Other insulating layers of silsesquioxane polymers are formed in the following references:
U.S. Pat. No. 4,670,299, issued to Fukuyama et al. on Jun. 2, 1987, prepares a lower alkyl polysilsesquioxane by dissolving a lower alkyltrifunctional silane in an organic solvent at a temperature of -20.degree. to -50.degree. C. to form a solution, dropping water into the solution under an inert gas pressurized at 1,000 to 3,000 Pa, and gradually heating the solution to a temperature of 60.degree. to 100.degree. C.
U.S. Pat. No. 4,801,507, issued to Estes et al. on Jan. 31, 1989, reacts an aryltrifunctional silanetriol (e.g. phenyltrichlorosilane) with a tetrafunctional silicic acid compound (e.g. ethyl silicate) to form a polymer rich in siloxane units.
For a variety of reasons, none of the foregoing techniques has proven to be entirely satisfactory.
In order to obtain an improvement in planarizing properties, the method in U.S. Pat. application Ser. No. 276,597, filed by Clodgo et al. on Nov. 28, 1988, now U.S. Pat. No. 4,981,530, forms an insulating layer by first preparing a solution by reacting water with an aminoalkoxysilane monomer in a solvent, by employing a critical mole ratio of water/monomer ranging from about 1.3/1-1.7/1. After the solution is allowed to sufficiently age, it is coated on a suitable substrate, which is then heated in an essentially oxygen-free atmosphere and under such conditions as to form a layer of cured ladder-type silsesquioxane polymer. In the practice of the method, an insulating layer is produced, which can achieve a high degree of planarization with improved crack resistance, however, in many applications, it is desirable to produce an insulating layer which demonstrates, not only desirable planarization characteristics, but also enhanced thermal stability.